FindMatlab: Fix not finding recent MCR installations

Apparently, newer MCR installations are not installed in a versioned subdiectory of the MATLAB root (i.e. Matlab Compiler Runtime/v84), but directly into the path provided by the registry. This MR changes FindMatlab.cmake to only consider that versioned subdirectory if it exists.

Also, newer MCR installations may not provide mex.h used for finding the MATLAB include directory. This MR adds matrix.h to the list of searched header files.

Fixes: #26306 (closed)
Backport: release
Topic-rename: FindMatlab-mcr